LOW-SHRINKAGE REFRACTORIES BONDED WITH HIGH-ALUMINA CEMENT MIXED WITHCOLLOIDAL SILICA

Low-shrinkage refractories were prepared by bonding ceramic grains wit h high-alumina cement mixed with colloidal silica. The phases formed d uring hydration or heat treatment were studied by XRD and IR-spectrosc opy. The main hydrated phases were CaO-Al2O3-10H(2)O (hydration at 20 degrees C) and 3CaO-Al2O3-6H(2)O plus 2CaO-Al2O3-SiO2-8H(2)O (hydratio n at 50 degrees C). 12CaO-7Al(2)O(3) was the first phase that crystall ised during thermal treatments. At T> 1000 degrees C, 12CaO-7A1(2)O(3) reacts with silica to form anorthite (CaO-Al2O3-2SiO(2)) and gehlenit e (2CaO-Al2O3-SiO2). Part of colloidal silica combines with calcium an d aluminum to form cristobalite, which is stabilised in its P -form. S hrinkage was measured by dilatometry. The crystallisation phenomena ar e beneficial to reduce firing shrinkage. The stabilisation of beta-cri stobalite eliminates the dilatometric anomaly usually induced by the b eta <---->alpha transformation at approximate to 200 degrees C.